In the surgical wound classification of the CDC Guidelines, the surgical field in middle ear surgery is often classified as “clean-contaminated” or “contaminated”. We investigated the occurrence of postoperative infections, including surgical site infections (SSIs), by performing bacteriological examinations immediately after surgery in patients who underwent middle ear surgery in our hospital. The investigation was conducted in 67 patients (29 males and 38 females) who underwent tympanoplasty between October 2005 and March 2007. As an antibacterial agent, PIPC was administered twice daily for 5 days starting immediately before to after surgery. Bacteriological examinations were performed using ear discharge specimens collected preoperatively and also using specimens collected by draining the mastoid antrum on the 1st to 3rd postoperative days and specimens scraped from the external ear canal on the 5th postoperative day. As a result, in most patients the microorganisms identified preoperatively were not detected in the mastoid antrum by the 3rd postoperative day. In the present clinical research, there were 5 patients judged to have developed SSI and 1 patient with a nonsurgical site infection (pneumonia). Since the mean hospitalization period for patients without SSI in our hospital was 10.9 days, it is considered that local and systemic conditions become stable within about 1 week after surgery.